So called "constant contact side bearings" have been provided for many years as a means of suppression of truck hunting by increasing resistance to truck bolster swivel. Although known devices work when initially installed, the effective life is relatively short. The need for frequent adjustment and replacement has limited the gains achieved through their use. Further, improvements in the control of car rocking through the use of side bearings have been generally quite limited because the existing devices commonly in use do not dissipate energy in the vertical direction. Where elastomeric springs are used for this purpose, any improvement achieved is short lived as these springs do not maintain their vertical stiffness on a long-term basis. This is due to the temperature rise associated with the constant rubbing which causes deterioration of the elastomeric material. In efforts to compensate for this problem, the initial vertical load carried by the side bearings is made relatively large but this tends to create a derailment hazard when the side bearings are used under empty cars. Side bearings exemplary of the known prior art are disclosed in my U.S. Pat. Nos. 4,131,069 and 4,655,143 and in U.S Pat. No. 5,048,427.